The present invention relates to a new and improved construction of a twin-wire papermaking machine and also relates to a new and improved method of dewatering a paper web in a twin-wire papermaking machine.
In its more particular aspects, the present invention specifically relates to a new and improved construction of a twin-wire papermaking machine containing a top or upper wire or screen and a bottom or lower wire or screen, each of which is guided in an endless loop at related guide rolls. The endless loops have a common section in which the top and bottom wires or screens form a twin-wire arrangement which passes at least one sheet or web forming roll. A paper web is formed between the twin-wire arrangement from a fiber stock suspension delivered by a headbox and such formed paper web is guided to a pressing roll at which the paper web is dewatered.
In a twin-wire or double-screen papermaking machine as described, for example, in U.S. Pat. No. 4,116,763, granted Sept. 26, 1978, and U.S. Pat. No. 4,176,005, granted Nov. 27, 1979, the sheet or web forming rolls are preferably constructed as open rolls. The paper web, after separation of the two wires or screens, is withdrawn in an approximately horizontal direction either from the top or upper wire or from the bottom or lower wire. Thereafter, the paper web, if desired, with the interposition of a take-off roll, is transferred to a felt web and fed to a pressing section which generally contains one or a number of suction rolls. In such arrangement a pressing roll cooperates with at least one, generally with a number of counter rolls. The paper web is thus dewatered under the action of a line-shaped pressing force and such dewatering operation is assisted by the felt web. As a result, the paper web can be subsequently fed to a drying section of the papermaking machine, and this drying section is provided consecutive to the pressing section.
In the presently required high-speed operation of such papermaking machines, i.e. at high travel speeds of the paper web, the employment of suction rolls as well as the series-arrangement of a number of pressing nips hitherto has been deemed indispensable in order to obtain safe web guidance and a sufficiently efficient mechanical dewatering process in the pressing section. However, suction rolls are expensive and cost-intensive machine elements in terms of manufacture as well as in terms of maintenance. Furthermore, a wire or screen section, the pressing section and the drying sections had to be series-arranged in the known papermaking machines, with the undesirable result that there prevailed considerable total or overall dimensions of the papermaking machine.